Well Test Analysis in Gas-Condensate Reservoirs

Abstract

Abstract Published analyses of well tests in gas-condensate reservoirs when pressure drops below the dew point are usually based on a two-zone radial composite model, representing regions of condensate drop-out around the wellbore and of initial gas composition away from the well. Laboratory experiments, on the other hand, suggest that three different mobility zones could exist:an outer zone away from the well, with the initial liquid condensate saturation;a zone nearer to the well, with increased condensate saturation and lower gas mobility; anda zone in the immediate vicinity of the well with high capillary number which increases the gas relative permeability, resulting in a recovery of much of the gas mobility lost from condensate blockage. This paper investigates the existence of this latter zone in well test data. An example of well test analysis is discussed, which illustrates the difficulty of identifying such a zone as, in many cases, build-up and/or drawdown data are dominated by wellbore phase redistribution effects. Where the three zones can be identified, data are analyzed using a three-zone radial composite model to yield a complete characterization of the near-wellbore effects, and in particular the knowledge of the various components of the total skin effect: mechanical skin; rate-dependent two-phase skin; and skin due to gas condensate blockage. The existence of the three zones and the results of the analysis are verified with a compositional simulator where relative permeability depends on capillary number. Introduction Gas condensate reservoirs exhibit a complex behavior due to the existence of a two-fluid system, reservoir gas and liquid condensate1–4. Three main problems are caused by liquid dropout when wells are produced below the dew point, namely: a non-reversible reduction in well productivity; a less marketable gas; and condensate-blocked pipelines. Consequently, many laboratory5,6,11,12,33theoretical1,2,4,9–14and field investigations10, 15–23have been conducted over the last forty years to try to understand condensate reservoir flow behavior. It has been found that, when reservoir pressure around a well drops below the dew point pressure, retrograde condensation occurs and three regions are created with different liquid saturations14,24,25. Away from the well, an outer region has the initial liquid saturation; next, there is an intermediate region with a rapid increase in liquid saturation and a corresponding decrease in gas relative permeability. Liquid in that region is immobile. Closer to the well, an inner region forms where the liquid saturation reaches a critical value, and the effluent travels as a two-phase flow with constant composition (the condensate deposited as pressure decreases is equal to that flown towards the well). There may also exist a fourth region in the immediate vicinity of the well where low interfacial tensions at high rates yield a decrease of the liquid saturation and an increase of the gas relative permeability1,9. The first, third and fourth regions should appear as three different permeability zones in a well test. The existence of the fourth region is particularly important as it would counter the reduction in productivity due to liquid dropout. This "velocity stripping26' has been inferred from laboratory experiments and numerical simulations but there has been little evidence of it from well test data published to date.